Embodiments of the present invention relate to the generation of electron beams and their applications.
Electron beams are used in a number of different applications. For example, electron beams can be modulated and directed onto an electron sensitive resist on a workpiece, such as a semiconductor wafer or mask, to generate an electron beam pattern on the workpiece. Electron beams can also be used to inspect a workpiece by, for example, detecting electrons emerging or reflected from the workpiece, to detect defects, anomalies or undesirable objects. Electron beams can also be used to irradiate a workpiece, such as a postal envelope, to destroy toxic chemicals or harmful microorganisms therein.
A typical electron beam apparatus comprises an electron beam column that includes an electron beam source to generate one or more electron beams and electron beam elements to focus or deflect the electron beams across a workpiece, which is held on a movable support. The electron beam source typically comprises a photocathode and wavelength matched radiation beam source. The photocathode can be a radiation transparent workpiece coated with an electron-emitting material. The electron-emitting material has an electron work function, which is the minimum electron emission energy level required to emit an electron from the surface of the material. A beam source directs radiation onto the backside of the transparent workpiece, the radiation having an energy level that is at least as high as the electron work function. When photons of the beam impinge on the electron-emitting material they excite electrons to a suitable energy level that emits the electrons from the electron-emitting material. For example, one photocathode-laser combination comprises an argon-ion laser, a frequency multiplier crystal, and a photocathode comprising a electron-emitting material of Mg or MgO. The argon-ion laser has a fundamental wavelength of 514 nm, which is reduced to 257 nm by the frequency multiplier crystal, to generate a laser beam having an energy level of about 4.8 eV. The frequency multiplied 4.8 eV laser beam has a higher energy than the workfunction of the Mg or MgO electron-emitting material, which is 3 to 4 eV, thus, the laser system and the electron-emitting material are suitably matched.
Electron-emitting materials used in such conventional photocathode systems have several limitations. For example, electron-emitting magnesium gradually oxidizes from exposure to residual oxygen in a low-pressure environment. MgO emitters often gives rise to deleterious blanking effects when the incident laser beam is blanked, i.e., turned on after an off period, when modulating the electron beams. In another example, the emission spot of a CsTe electron-emitting material often grows in size in operation, requiring the electron-emitting material to be patterned or covered with a protective anti-oxidation layer of CsBr, as described in commonly assigned U.S. patent application publication no. US 2003/0042434A1, which is incorporated herein by reference in its entirety. Cesium antimonide electron-emitting materials also have to be covered with a protective layer of CsBr to minimize attenuation of the quantum efficiency of the electron-emitting over time in an oxygen environment, as described in U.S. Pat. No. 6,531,816 B1, which is also incorporated herein by reference in its entirety.
A further problem with conventional electron beam apparatus arises from their throughput versus resolution trade-off. Conventional apparatus that use a single electron beam to scan across a workpiece provide relatively low throughput when used at high resolutions. For example, at current line width resolutions of 100 to 130 nm, a single electron beam system takes about 6 hours to scan across the entire surface of a 200 mm workpiece; however, at resolutions of 35 to 50 nm, the same system would take about 50 hours to scan the same workpiece. This problem is reduced in multiple electron beam apparatus, which use a plurality of electron beams drawn from one or more electron sources as separate and well-defined beams. The multi-beam systems provide higher throughput and speed even at high resolutions. However, even these multi-beam systems are limited by the degradation, low beam current and electron cross-over limitations of conventional photocathodes.
Thus, it is desirable to have an electron generating system that can generate a consistent stream of electrons without deleterious changes in operation. It is further desirable to have a properly matched photocathode and beam source capable of generating electrons with good efficiency and consistent emissivity. It is also desirable to have a stable photocathode that does not degrade due to oxidation in the vacuum environment. It is further desirable to have an electron beam apparatus capable of providing good throughput at high resolutions.